1. Field of the Invention
This invention relates to packaged micromachined devices such as vacuum micropump devices, devices having a micromachined sealed electrical interconnect and devices having a suspended micromachined bonding pad.
2. Background Art
The following references are noted herein:                [1] R. A. Miller et al., “A MEMS Radio-Frequency Ion Mobility Spectrometer for Chemical Vapor Detection,” SENS. ACTUA., A91, 301 (2001).        [2] C. Wilson et al., “Silicon Micro-machining Using In-Situ DC Microplasmas,” J. MICROELECTROMECH. SYST., 10(1), 50 (2001).        [3] F. Iza et al., “Influence of Operating Frequency and Coupling Coefficient on the Efficiency of Microfabricated Inductively Coupled Plasma Sources,” PLASMA SOURCES SCI. TECHNOL., 11, 1 (2002).        [4] C. G. Wilson et al., “Spectral Detection of Metal Contaminants in Water Using an On-Chip Microglow Discharge,” IEEE TRANSACTIONS ON ELECTRON DEVICES, 49(12), 2317-2322 (2002).        [5] J. P. Hobson et al., “Review of Pumping by Thermal Molecular Pressure,” J. VAC. SCI. TECHNOL., A18(4), 1758 (2000).        [6] M. Knudsen, ANNALS DER PHYSIK, 31, 205 (1910).        [7] J. P. Hobson, “Accommodation Pumping—A New Principle,” J. VAC. SCI. TECHNOL., 7(2), 351 (1970).        [8] D. H. Tracey, “Thermomolecular Pumping Effect,” J. PHYS. E: SCI. INSTR., 7, 533 (1974).        [9] E. P. Muntz et al., “Microscale Vacuum Pumps,” THE MEMS HANDBOOK, M. Gad-el-Hak, Ed. (CRC Press, Boca Raton, 2002), Chap. 29.        [10] D. J. Turner, “A Mathematical Analysis of a Thermal Transpiration Vacuum Pump,” VACUUM 16(8), 413 (1966).        [11] C. C. Wong et al., “Gas Transport by Thermal Transpiration in Micro-Channels—A Numerical Study,” PROCEEDINGS ASME MEMS CONFERENCE, DSC-Vol. 66 (Anaheim, Calif., 1998), pp. 223-228.        [12] R. M. Young, “Analysis of a. Micromachine Based Vacuum Pump on a Chip Actuated by the Thermal Transpiration Effect,” J. VAC. SCI. TECHNOL., B17(2), 280 (1999).        [13] S. E. Vargo et al., “Knudsen Compressor as a Micro- and Macroscale Vacuum Pump Without Moving Parts or Fluids,” J. VAC. Sci. TECHNOL., A17(4), 2308 (1999).        [14] S. E. Vargo et al., “Initial Results From the First MEMS Fabricated Thermal Transpiration-Driven Vacuum Pump,” RAREFIED GAS DYNAMICS: 22ND INTL. SYMP., p. 502 (2001).        [15] E. Kennard, “Kinetic Theory of Gases,” (McGraw Hill, New York, 1938).        [16] S. McNamara et al., “A Fabrication Process with High Thermal Isolation and Vacuum Sealed Lead Transfer for Gas Reactors and Sampling Microsystems,” PROCEEDINGS IEEE THE SIXTH ANNUAL INTERNATIONAL CONFERENCE ON MICRO ELECTRO MECHANICAL SYSTEMS, (IEEE 2003), pp. 646-649.        [17] S. McNamara et al., “A Micromachined Knudsen Pump for On-Chip Vacuum,” DIGEST OF TECHNICAL PAPERS OF THE 12TH INTERNATIONAL CONFERENCE ON SOLID-STATE SENSORS AND ACTUATORS, 2003, pp. 1919-1922.        
Micromachined gas pumps have a variety of potential applications, ranging from actuation of gases for gas chromatography, spectroscopy [1], or microplasma manufacturing [2,3], to the pneumatic actuation of liquids for lab-on-a-chip and chemical sensing devices [4]. Conventional vacuum pumps scale down poorly due to increased surface-to-volume ratio and have reliability concerns due to the relative increase of frictional forces over inertial forces at the microscale. Thermal molecular pumps can potentially overcome these challenges. There are three types of thermal molecular pumps [5]: the Knudsen pump [6], the accommodation pump [7], and the thermomolecular pump [8]. The Knudsen pump exploits the temperature dependence of molecular flux rates through a narrow tube; the accommodation pump exploits the temperature dependence of the tangential momentum accommodation coefficient (TMAC) of gases; whereas the thermomolecular pump exploits some materials that violate the cosine scattering law when heated.
The Knudsen pump provides the highest compression ratio and, unlike the other two pumps, its performance is independent of the material and surface conditions, which can be difficult to characterize and control. A miniaturized Knudsen pump also has a high theoretical efficiency when compared to conventional vacuum pumps [9] and scales well to small dimensions because the efficiency improves as the surface-to-volume ratio increases. It offers potentially high reliability because there are no moving parts, but power consumption can be a major concern because of the elevated temperatures required.
The Knudsen pump was first reported in 1910 and since then has been reported approximately once per decade [10]. Despite its attractive features, persistent challenges that have prevented its widespread adoption include the need for sub-micron dimensions to operate at atmosphere (and consequently it was always confined to high vacuum operation over a limited pressure range) and low throughput. The past decade has witnessed greater activity, with simulation efforts [11,12] and a partially micromachined implementation achieving a best-case pressure drop of 11.5 Torr using helium [13,14].
The Knudsen Pump Theory
The principle of thermal transpiration [15], on which the Knudsen pump is based, describes the pressure-temperature relationship between two adjacent volumes of gas at different temperatures. If these two volumes of gas are separated by a channel or aperture that permits gas flow only in the free molecular regime (FIG. 1), they settle at different pressures, the ratio of which is a function of only temperature. The temperature difference does not create a pressure difference between the chambers with a channel that permits viscous flow.
The following patent references are related to the present invention: U.S. Pat. Nos. 6,533,554 and 5,871,336 and published U.S. patent application Ser. No. 2001/0003572.
The following references are also noted herein:                [A] C. Zhang et al., “An Integrated Combustor-Thermoelectric Micro Power Generator,” TECHNICAL DIGEST, DIGEST, TWELFTH IEEE CONF. ON SOLID-STATE SENSORS AND ACTUATORS (Transducers '01), Munich, Germany, pp. 34-37, June 2001.        [B] C. Zhang et al., “Fabrication of Thick Silicon Dioxide Layers Using DRIE, Oxidation and Trench Refill,” TECHNICAL DIGEST, IEEE 2002 INT. CONF. ON MICRO ELECTRO MECHANICAL SYSTEMS, (MEMS 2002), Las Vegas, pp. 160-163, January 2002.        [C] C. M. Yu et al., “A High Performance Hand-Held Gas Chromatograph,” ASME PROC. OF MICROELECTROMECHANICAL SYSTEMS, (MEMS), 1998, Anaheim, Calif., pp. 481-6.        [D] Y. T. Cheng et al., “Vacuum Packaging Technology Using Localized Aluminum/Silicon-to-Glass Bonding,” PROC. IEEE INTL. MEMS CONF., 2001, Interlaken, Switzerland, pp. 18-21.        [E] A. V. Chavan et al., “Batch-Processed Vacuum-Sealed Capacitive Pressure Sensors,” J. MICROELECTROMECHANICAL SYSTEMS, v. 10(4), 2001, pp. 580-588.        
In recent years, there has been substantial interest in developing gas-handling Microsystems that can serve as reactors, combustors, and detectors such as mass spectrometers [A-C]. These systems, which may also integrate pumps, reservoirs, flow sensors, and pressure sensors, often operate at elevated temperatures and require high thermal isolation for energy efficiency and minimization of cross-talk. In addition, when capacitive transducers are used, a vacuum-sealed lead transfer with low parasitic capacitance is a significant asset. While there have been strong efforts on vacuum micropackaging [D] and sealed lead transfer [E], research has not been directed at simultaneously achieving high thermal isolation and sub-femtofarad parasitic capacitance.
For many transducers that operate in vacuum, the ability to create and control vacuum within an on-chip cavity promises enhanced performance, longer lifetime, and simplified packaging. While locally heated getter materials can maintain a vacuum in microcavities, as described in published U.S. patent application Ser. No. 2003/0089394, they are unsuitable for systems that continuously sample gases.